The present invention relates generally to the field of image quality assessment of medical imaging systems and, more particularly, to a phantom and method of assessing imaging quality of a digital breast tomosynthesis system.
Breast cancer remains one of the most common cancers among women despite early detection methods such as breast self-examination, annual mammograms and clinical breast exams. The objective of screening mammography is to identify cancers while they are small and localized. However, some breast cancers still are missed while in their earliest stages, and it is widely agreed that imaging of dense breasts can be improved.
In conventional two dimensional film or full field digital mammography (FFDM), overlapping dense fibroglandular tissue within the breast can reduce the visibility of malignant abnormalities or simulate the appearance of an abnormality. This can lead to unnecessary patient recalls, biopsies and psychological stress. In addition, mammography is known to be less sensitive in women with dense breasts, who are at higher risk of developing breast cancer.
Digital breast tomosynthesis (DBT), also referred to as three-dimensional (3D) mammography, is a newly developed form of three-dimensional imaging with the potential to improve the accuracy of mammography by reducing tissue overlap. This overlap, which is sometimes known as anatomical noise, degrades image quality in standard 2D projection imaging and can mask suspicious areas.
In DBT, the breast is compressed, in the standard fashion, between a compression paddle or plate and a breast support plate overlying an image receptor/detector. With the breast kept stationary, an x-ray tube is moved in an arcuate or isocentric motion and a series of low-dose images, known as projections, are taken at different angular locations of the tube over a limited total angular range known as the scan angle. The projections undergo a reconstruction process using algebraic reconstruction algorithms to produce high resolution images, also known as tomographic sections or slices, in planes parallel to the breast support plate. The number of reconstructed slices will depend upon the thickness of the compressed breast and the desired separation between slices, which might typically be around 1 mm. These sections can be viewed on a work station, either as individual slices or sequentially in a dynamic video mode. By minimizing the superimposition of overlying breast tissue, DBT has the potential to differentiate malignant features more definitively from non-malignant ones.
Removal of confusing overlying tissue provides clearer imaging, better sensitivity and fewer patient recalls with DBT. Digital breast tomosynthesis also has the potential for lower radiation dosage and less breast compression. Significantly, by detecting breast cancer early, a woman's chances of survival are higher and she may have more treatment options available to her.
Tomosynthesis differs from computed tomography (CT) in several significant aspects. In DBT, projections are obtained over only a limited range of angles, while in CT, projections are obtained through either 180° or 360° rotations of x-ray tubes and detectors.
In conventional computed tomography (CT), a plane of interest is established by moving the detector and the x-ray tube in opposite directions. This establishes a plane of interest (or a plane of focus). Features within the plane of focus appear relatively sharp. A major disadvantage of this approach is that there is only a single plane of focus for each exposure and geometric configuration. Another disadvantage of CT imaging of the breast is the higher radiation dose involved in conventional system designs.
In contrast, in tomosynthesis, an arbitrary number of planes may be retrospectively reconstructed from a single sequence of projection images. Typically, a series of projection images is obtained while the x-ray tube moves in a limited arcuate or linear motion. (However, the motion of the x-ray tube could be more complex and the imaging detector could be stationary or moving.) After the acquisition sequence is complete, the projection images are combined by shifting and adding these together to bring a specific plane into focus. Different planes can be brought into focus by varying the amount of shifting. Advantages of tomosythesis over conventional projection imaging include: depth localization, improved conspicuity owing to the removal of the clutter caused by overlying tissue structures, and improved contrast of local structure by limiting the dynamic range to a single plane.
The differences between tomosynthesis and CT impose different requirements on phantoms and methods of assessing image quality in the two different imaging modalities. As opposed to relatively well defined slices with finite extension in CT, tomosynthesis slices are less well defined with some degree of extension from the center of the slice of interest to the boundaries of the object being considered. Due to their disparate methods of reconstruction, artifacts and image problems are very different in tomosynthesis as compared to CT, and new and different approaches to the measurement of image quality in tomosynthesis are needed.
In a recent study, entitled “Evaluation of Various Mammography Phantoms for Image Quality Assessment in Digital Breast Tomosynthesis” by Claudia Brunner et al, four different existing mammography phantoms were investigated for their appropriateness for image quality evaluation in Digital Breast Tomosynthesis. This study concluded that, “Although each phantom under study has its advantages, none of them allows a thorough quality evaluation of reconstructed tomosynthesis images”. It can also be noted from this study, that modulation transfer function (MTF) data is only available from transforming edge response function in one direction at a time, and no point source for 2D information and 2D MTF transform is available. Cited deficiencies in all phantoms studied include slice sensitivity profile.
In a “Protocol for the Quality Control of the Physical and Technical aspects of Digital Breast Tomosynthesis Systems”, draft version 0.10, published in February 2013, by the European Reference Organisation for Quality Assured Breast Screening and Diagnostic Services (EUREF), a phantom for z-resolution in DBT is disclosed that comprises a planar array of 25 spaced apart aluminum spheres. In use, this phantom, must be repositioned and exposed at multiple different heights in a time consuming and repetitive procedure.
The present invention provides a phantom and method of image quality assessment specifically tailored and optimized for digital breast tomosynthesis systems.